The present invention generally relates to devices of reversibly variable transmittance to electromagnetic radiation. More specifically, the present invention relates to an improved electrode design for electrochromic light filters and mirrors.
Devices of reversibly variable transmittance to electromagnetic radiation have been proposed as the variable transmittance element in variable transmittance light filters, variable reflectance mirrors, and display devices, which employ such light filters or mirrors in conveying information. These variable transmittance light filters have included architectural windows, skylights, and windows and sunroofs for automobiles.
Devices of reversibly variable transmittance to electromagnetic radiation, wherein the transmittance is altered by electrochromic means, are described, for example, by Chang, xe2x80x9cElectrochromic and Electrochemichromic Materials and Phenomena,xe2x80x9d in Non-emissive Electrooptic Displays, A. Kmetz and K. von Willisen, eds. Plenum Press, New York, N.Y. 1976, pp. 155-196 (1976) and in various parts of Electrochromism, P. M. S. Monk, R. J. Mortimer, D. R. Rosseinsky, VCH Publishers, Inc., New York, N.Y. (1995). Numerous electrochromic devices are known in the art. See, e.g., U.S. Pat. No. 3,451,741 issued to Manos; U.S. Pat. No. 4,090,358 issued to Bredfeldt et al.; U.S. Pat. No. 4,139,276 issued to Clecak et al.; U.S. Pat. No. 3,453,038 issued to Kissa et al.; U.S. Pat. Nos. 3,652,149, 3,774,988, and 3,873,185 issued to Rogers; and U.S. Pat. Nos. 3,282,157, 3,282,158, 3,282,160, and 3,283,656 issued to Jones et al.
In addition to these devices, there are commercially available electrochromic devices and associated circuitry, such as those disclosed in U.S. Pat. No. 4,902,108, entitled xe2x80x9cSINGLE-COMPARTMENT, SELF-ERASING, SOLUTION-PHASE ELECTROCHROMIC DEVICES, SOLUTIONS FOR USE THEREIN, AND USES THEREOF,xe2x80x9d issued Feb. 20, 1990, to H. J. Byker; Canadian Patent No. 1,300,945, entitled xe2x80x9cAUTOMATIC REARVIEW MIRROR SYSTEM FOR AUTOMOTIVE VEHICLES,xe2x80x9d issued May 19, 1992, to J. H. Bechtel et al.; U.S. Pat. No. 5,128,799, entitled xe2x80x9cVARIABLE REFLECTANCE MOTOR VEHICLE MIRROR,xe2x80x9d issued Jul. 7, 1992, to H. J. Byker; U.S. Pat. No. 5,202,787, entitled xe2x80x9cELECTRO-OPTIC DEVICE,xe2x80x9d issued Apr. 13, 1993, to H. J. Byker et al.; U.S. Pat. No. 5,204,778, entitled xe2x80x9cCONTROL SYSTEM FOR AUTOMATIC REARVIEW MIRRORS,xe2x80x9d issued Apr. 20, 1993, to J. H. Bechtel; U.S. Pat. No. 5,278,693, entitled xe2x80x9cTINTED SOLUTION-PHASE ELECTROCHROMIC MIRRORS,xe2x80x9d issued Jan. 11, 1994, to D. A. Theiste et al.; U.S. Pat. No. 5,280,380, entitled xe2x80x9cUV-STABILIZED COMPOSITIONS AND METHODS,xe2x80x9d issued Jan. 18, 1994, to H. J. Byker; U.S. Pat. No. 5,282,077, entitled xe2x80x9cVARIABLE REFLECTANCE MIRROR,xe2x80x9d issued Jan. 25, 1994, to H. J. Byker; U.S. Pat. No. 5,294,376, entitled xe2x80x9cBIPYRIDINIUM SALT SOLUTIONS,xe2x80x9d issued Mar. 15, 1994, to H. J. Byker; U.S. Pat. No. 5,336,448, entitled xe2x80x9cELECTROCHROMIC DEVICES WITH BIPYRIDINIUM SALT SOLUTIONS,xe2x80x9d issued Aug. 9, 1994, to H. J. Byker; U.S. Pat. No. 5,434,407, entitled xe2x80x9cAUTOMATIC REARVIEW MIRROR INCORPORATING LIGHT PIPE,xe2x80x9d issued Jan. 18, 1995, to F. T. Bauer et al.; U.S. Pat. No. 5,448,397, entitled xe2x80x9cOUTSIDE AUTOMATIC REARVIEW MIRROR FOR AUTOMOTIVE VEHICLES,xe2x80x9d issued Sep. 5, 1995, to W. L. Tonar; and U.S. Pat. No. 5,451,822, entitled xe2x80x9cELECTRONIC CONTROL SYSTEM,xe2x80x9d issued Sep. 19, 1995, to J. H. Bechtel et al. Each of these patents is commonly assigned with the present invention and the disclosures of each, including the references contained therein, are hereby incorporated herein in their entirety by reference. Such electrochromic devices may be utilized in a fully integrated inside/outside rearview mirror system for a vehicle or as separate inside or outside rearview mirror systems.
It is desirable to use reversibly variable transmittance light filters in architectural windows, skylights, and in windows and sunroofs for automobiles in order to reduce the transmittance of the filter with respect to direct or reflected sunlight during daytime, while not reducing such transmittance during nighttime. Not only do such light filters reduce bothersome glare and ambient brightness, they also reduce fading and generated heat caused by the transmittance of sunlight through the window.
Variable transmission electrochromic devices such as windows and light filters typically include a structure similar to that shown in FIG. 1. Specifically, they typically include first and second transparent substrates 12 and 14, which are commonly made of glass and arranged in parallel, spaced-apart relation. The electrochromic devices also typically include first and second transparent, electrically conductive layers forming electrodes 16 and 18 provided on the interfacing surfaces of substrates 12 and 14. A seal 20 is provided to secure the coated substrates together and to provide a chamber 22 between the coated substrates in which an electrochromic medium 24 is provided. Electrically conductive clips 26 and 28 are respectively attached to one of the coated substrates so as to be electrically coupled to one of electrode layers 16 and 18. The electrochromic medium 24 is contained in chamber 22. The electrochromic medium 24 is in direct contact with transparent electrode layers 16 and 18, through which passes electromagnetic radiation whose intensity is reversibly modulated in the device by a variable voltage or potential applied to electrode layers 16 and 18 through clip contacts 26 and 28 and an electronic circuit (not shown).
The electrochromic medium 24 includes two different coloring speciesxe2x80x94a cathodic species and an anodic species, which are colorless or nearly colorless in an inactivated state. In most cases, when there is no electrical potential difference between transparent electrodes 16 and 18, the electrochromic medium 24 in chamber 22 is colorless or nearly colorless, and incoming light (Io) enters through second substrate 14, passes through transparent electrode 18, electrochromic containing chamber 22, transparent electrode 16, and first substrate 12. When a potential difference is applied between transparent electrodes 16 and 18 at the cathode, the cathodic species are reduced (i.e., accept electrons from the cathode). On the other hand, the anodic species are oxidized at the anode (i.e., donate electrons to anode 16). As the cathodic and anodic species in electrochromic medium 24 accept and donate electrons from/to electrodes 18 and 16, respectively, at least one of the species becomes colored. The anodic and cathodic species in medium 24 return to a colorless or nearly colorless state once they exchange electrons in the center portion of chamber 22. Nevertheless, so long as a sufficient potential is applied across electrodes 16 and 18, there is a sufficient amount of the anodic and cathodic species that are oxidized and reduced so as to color an electrochromic cell. Because the anodic and cathodic species exchange electrons in the center portion of chamber 22 and donate and accept electrons when adjacent a respective electrode 16 and 18, the reduced cathodic component contributing to the perceived color exists primarily adjacent cathode 18, and the oxidized anodic component exists proximate to anode 16. This also corresponds to the fact that the concentration of reduced cathodic species is greatest proximate cathode 18, and the concentration of oxidized anodic species is greatest adjacent anode 16.
Commercially available electrochromic media that are suitable for use in chamber 24 generally include solution-phase and solid-state electrochromic materials. In an all solution-phase medium, the electrochemical properties of the solvent, optional inert electrolyte, anodic materials, cathodic materials, and any other components that might be present in the solution are preferably such that no significant electrochemical or other changes occur at a potential difference which oxidizes anodic material and reduces the cathodic material other than the electrochemical oxidation of the anodic material, electrochemical reduction of the cathodic material, and the self-erasing reaction between the oxidized form of the anodic material and the reduced form of the cathodic material.
Electrode layers 16 and 18 are connected to electronic circuitry which is effective to electrically energize the electrochromic medium, such that when a potential is applied across the transparent electrodes 16 and 18, electrochromic medium 24 in chamber 22 darkens, such that incident light (Io) is attenuated as the light passes through the electrochromic device. By adjusting the potential difference between the transparent electrodes, such a device can function as a xe2x80x9cgray-scalexe2x80x9d device, with continuously variable transmittance over a wide range. For solution-phase electrochromic systems, when the potential between the electrodes is removed or returned to zero, the device spontaneously returns to the same zero-potential, equilibrium color and transmittance as the device had before the potential was applied.
Another common construction for an electrochromic device is shown in FIG. 2. In the construction shown in FIG. 2, the first and second transparent substrates 12 and 14 are arranged in a parallel, spaced-apart relationship in the same manner as the electrochromic device shown in FIG. 1. Also, a seal 20 is provided between substrates 12 and 14 so as to provide a sealed chamber 22 lying therebetween. The electrochromic device shown in FIG. 2 differs from that shown in FIG. 1 in that electrochromic medium 24 is solid-state rather than a solution-phase and is formed within a multi-layer stack, with an electrolyte material layer 30 adjacent electrochromic layer 24 between first and second transparent electrode layers 16 and 18. This stack is carried on the inner surface of one of first and second substrates 12 and 14, with either gas or air surrounding the stack within chamber 22. As will be described further below, the electrochromic device shown in FIG. 2 is susceptible to many of the same problems as the electrochromic device shown in FIG. 1.
Electrochromic devices of the type described above are susceptible to irreversible damage from ultraviolet (UV) radiation when operating in its low transmission state. More specifically, the anodic and cathodic species in electrochromic medium 24 can be adversely and permanently affected by the UV light emitted by the sun when they are in their colored states. When the species are not in their colored states, they are generally not adversely affected by UV radiation. The UV absorption problem has made it impractical to use electrochromic light filters in window applications where it is desired to darken windows during daytime hours. To minimize the adverse impact of UV radiation on the electrochromic medium, UV absorbers are often introduced into the electrochromic medium. These absorbers absorb the UV radiation so as to minimize the amount of UV radiation that is absorbed by the anodic and cathodic species within the electrochromic medium. Such UV radiation absorbers, however, are not as effective when the electrochromic device is darkened, because of the fact that the colored species tend to be concentrated adjacent to anode 16 and cathode 18 and because these colored species thus tend to absorb the UV radiation before the UV absorber can absorb the UV radiation.
An additional problem with implementing electrochromic devices in windows is that the amount of current drawn by the type of electrochromic devices shown in FIGS. 1 and 2 is fairly substantial when the electrochromic devices are in their colored states. Also, color bands, or segregation, is known for devices colored for prolonged periods of time. These problems become very significant when electrochromic light filters are utilized in all the windows of a large building.
Yet another practical problem that arises when utilizing electrochromic light filters in architectural windows as well as other forms of windows is that such electrochromic devices are typically made with a relatively large cell spacing between the substrates in order to obtain uniform color throughout the electrochromic medium. Any variations in the distance between the anode and the cathode layers results in variations in color in the electrochromic medium. Tempered glass substrates are generally not very flat. Thus, when tempered glass is used for substrates 12 and/or 14, the need to create a thick cell in a window becomes more important.
Accordingly, it is an aspect of the present invention to overcome the above-noted problems associated with implementing an electrochromic device as a variable transmittance light filter. More specifically, it is an aspect of the present invention to provide an electrochromic device that is less susceptible to damage from UV radiation. Another aspect of the present invention is to provide an electrochromic device that draws less operating current. Another aspect of the present invention is to provide an electrochromic device that is less likely to be affected by surface variations in the transparent substrates that define the chamber in which the electrochromic medium is disposed.
To achieve these and other aspects and advantages, the electrochromic device of the present invention comprises a transparent first substrate having an outer surface and an inner surface, a second substrate having an inner surface spaced apart from the inner surface of the first substrate so as to define a chamber therebetween, a first electrode carried on the inner surface of the second substrate, and a second electrode also carried on the inner surface of the second substrate such that the second electrode is electrically isolated from the first electrode. The electrochromic device further includes an electrochromic medium disposed in the chamber id between the inner surface of the first substrate and the inner surface of the second substrate, which carries the first and second electrodes. The first and second electrodes may be disposed on the inner surface of the second substrate so as to be substantially co-planar with one another. Alternatively, the electrodes may be arranged in a stacked configuration, with a layer of dielectric material provided therebetween. In this alternative configuration, the dielectric layer and the second electrode would include a plurality of apertures through which the electrochromic medium may contact the first electrode, which underlies the dielectric and second electrode layers.
To achieve the above and other aspects and advantages, an electrochromic device may further be constructed in accordance with the present invention by having a positive electrode, a negative electrode having a surface area different from the surface area of the positive electrode, and an electrochromic medium including cathodic and anodic species, wherein the ratio of cathodic to anodic species within the electrochromic medium is a function of the ratio of the surface areas of the positive and negative electrodes.